Photocuring systems have been developed in which visible and/or ultraviolet light is used to induce curing, hardening, and/or polymerization of monomeric, oligomeric or polymeric materials. Generally speaking, a photocurable resin/adhesive includes a photoinitiator responsible for initiating free-radical polymerization of the resin. The resin remains in a liquid/workable condition until polymerization is initiated. In order to initiate polymerization, light source is used to provide light of a wavelength suitable for absorption by the photo initiator. The photoinitiator enters an excited state upon absorption of photons of the correct wavelength inducing the creation of free-radicals. The free-radicals induce curing, hardening, and/or polymerization of monomeric, oligomeric or polymeric resin/adhesive.
Light energy is typically provided by one of four types of curing lights: quartz-tungsten-halogen (QTH), arc lamps, light-emitting diode (LED), arc lamps, and argon laser. Both QTH and arc lamps have broad emission spectra suitable for initiating polymerization in a broad range of resins. However, QTH and arc lamps also emit a great deal of heat/infrared. The heat/infrared output is reduced utilizing filters which may also be used to select output wavelengths suitable for particular photoinitiators. The large heat output however requires the QTH and arc lamp systems to have substantial thermal management systems and also reduces the life span of the lamps such that costly replacement parts are required. Moreover, both QTH and arc lamps have significant warm-up periods before spectral output is stable. Thus, in practice the lamps must be kept running continuously while light output is controlled using a shutter. This further reduces the effective lifespan of the lights.
Argon laser systems can be used to provide light for photocuring applications. The light output is coherent and can thus be used to generate high intensity illumination with photons of a selected wavelength. However, the emission spectra of the Argon laser is very narrow and may not be compatible with some photocurable resins. Moreover, Argon laser systems are expensive and have significant thermal regulation requirements
LEDs (light-emitting diodes) have matured significantly within the last decades. LEDs emit light in specific wavelengths and generate much less heat relative to arc and QTH lamps thereby providing for longer lifespan, easy switching, consistent output and lower power consumption. However LEDs presents trade-offs with respect to emission wavelength dependent intensity, broad emission spectrum (spectral half width on the order of 30 nm or more), poor spectral stability, and the wide angular range of emission. The narrow band emission may not be compatible with some photocurable resins. In addition, the process used to manufacture LED's cannot tightly control their spectral stability; anyone wishing to use LED's in applications requiring a good spectral stability typically works directly with a supplier to essentially hand-pick the LED's for the particular application. Moreover the spectral output of an LED varies with temperature. Also, LED's emit light over a wide angular range (50% of light intensity emitted at) 70°). While optics can narrow the emission band and focus the light output, the resulting loss in power and increase in thermal output further complicates the use of LEDs for photocuring. Thus, it can be difficult to provide sufficient light at a wavelength suitable for exciting a particular photoinitiator.
While lighting manufacturers cannot provide all things to all applications, it is precisely this breadth of demand for which a light engine can be designed. To that end, products are not simple sources, but rather light engines, sources and all the ancillary components required to provide pure, powerful, light to the sample or as close to it as mechanically possible. A qualitative comparison of light engine performance as a function of source technology is summarized in Table I.
TABLE IA qualitative comparison of light source technology.SourceUseableTemporalHeatTechnologyLightUniformityResponseGenerationDurabilityCostArc LampmedpoornonehighlowhighLaserhighpoornonelowlowvery highLEDlowpoorfastlowhighmediumQTHlowpoornonemediumlowmediumLEDhighhighfastlowhighlow
A wide range of photoinitiators are available. To initiate polymerization it is essential to provide sufficient light energy at a wavelength which can be absorbed by a selected photoinitiator. However, each photoinitiator has a particular absorption spectra. Additionally, the light energy may have to pass through the resin and other materials in order to reach the photoinitiator. Resins incorporating a photoinitiator can affect transmission and absorption of light in different ways. Accordingly, it may be difficult to ensure sufficient light energy is provided at a wavelength suitable for exciting the photoinitiator. Without proper absorption, free radical polymerization may not occur uniformly throughout the resin. Moreover, where narrow band light sources, such as LEDs, are used the wavelength provided will not be suitable for exciting all photoinitiators in all compositions and manufacturing environments.
Accordingly it would be desirable to provide an LED light source for photocuring that overcomes limitations of the prior art.